Already known is a three-way catalytic converter, by which three major toxic components (HC, CO and NO.sub.x) in the gas exhausted from an internal combustion engine can be eliminated. The operation of this known three-way catalytic converter is effectively attained when an atmosphere of the exhaust gas introduced into the converter is controlled near stoichiometric atmosphere in which excess air or excess fuel is, substantially not left in the exhaust gas, i.e., the excess air ratio .lambda. of the exhaust gas is kept near 1.0.
However, it is impossible to keep the .lambda. near 1.0 in a known internal combustion engine, especially, in a carburetor type internal combustion engine; therefore, an effective operation of the three-way catalytic converter is not expected.
To eliminate this drawback, we have already proposed a system for introducing secondary air into an internal combustion engine, provided with a three-way catalytic converter arranged in the exhaust system of the engine, which system comprising a flow control valve having a diaphragm for forming a first chamber and a second chamber one on each side thereof, and which is connected to a valve member for controlling the amount of secondary air (Japanese Patent Application No. 049388/76), corresponding to U.S. Pat. No. 733,167. A vacuum signal is transmitted to the first or the second chamber in order to move the valve member so that the amount of the secondary air is increased or decreased in accordance with the atmosphere of the exhaust gas sensed by a .lambda.-sensor arranged in the exhaust system, whereby excess air ratio .lambda. is controlled near 1.0. The system further comprises delay units for restricting the transmission of the vacuum signal to the first and second chambers, so that the valve member is moved at a controlled rate for preventing a rapid ON-OFF operation of the valve.